Autonomic nervous system (ANS) is a multifunctional system regulated by the sympathetic nervous system and the parasympathetic system, providing a rapidly responding mechanism to control a wide range of bodily functions such as, for example, cardiovascular, respiratory, gastrointestinal, genitor-urinary, exocrine and endocrine secretions, and microcirculation. Furthermore, ANS is involved in the regulation of immune and inflammatory processes. Autonomic dysfunction may affect both the sympathetic nervous system and the parasympathetic nervous system and may affect any organ that is innervated by the autonomic nervous system.
Heart rate (HR) and heart rate variability (HRV) are affected by both internal and external changes in, for example, breathing, blood pressure, hormone status, mental condition and physical conditions. A number of pathophysiological conditions may shift the balance in the ANS thereby decreasing or increasing stimulation to heart's sinoatrial node which controls HR and HRV. For example, increase in blood pressure causes arteries to stretch, thereby causing increase in baroreceptor discharge frequency which, in turn, causes increase in parasympathetic and decrease in sympathetic activity. Similarly, carotid chemoreceptor stimulation by noradrenalin leads to slowing HR and increase in rate and depth of respiration.